In satellite/cellular systems, the signal bandwidth and channel spacing used in the satellite mode is considerably different from the signal bandwidth used in the cellular mode. For example, the GSM cellular system has a channel spacing of 200 KHz, while a satellite system may use a channel spacing of 5 KHz. In the narrow band mode, frequency and phase noise is considerably more troublesome than in the wideband mode. As a result, difficulties can arise in attempting to design re-used patterns for both modes. In analog cellular telephones such as the ones designed for the U.S. AMPS system, for example, conventional single-loop digital frequency synthesizers can be employed. In such cellular phones, the channel spacing is 30 KHz and there are no requirements for extremely fast channel changing. In the pan-European digital cellular system GSM, frequency hopping is employed to mitigate the effects of slow fading. Fast frequency switching synthesizers are then needed, but since the channel steps of 200 KHz are relatively coarse, the switching speed can be achieved by conventional techniques.
However, in the U.S. digital cellular system IS54, the channel spacing is the same as the channel spacing in the AMPS system at 30 KHz, but a requirement for fast frequency changing arises in order to permit mobile phones to scan the frequencies of surrounding base stations during short idle periods. U.S. patent application Ser. No. 07/804,609 which is commonly assigned and now U. S. Pat. No. 5,180,993, describes techniques that can be employed to assist in meeting this fast frequency changing requirement and is incorporated herein by reference. One of the techniques disclosed is called "fractional-N" which is a way to obtain free frequency steps while keeping the phase comparison frequency at which the loop phase error detector operates relatively high. This is desirable when fast switching and low phase noise must be achieved.
In "Phaselock techniques" by Floyd M. Gardener, Wiley 1979, Gardener a dual-loop synthesizer called "vernier loop" is illustrated which permits the generation of small frequency steps while keeping the phase comparison frequency of both loops high. This prior technique is an alternative technique for achieving small frequency steps, fast switching and low phase noise. Using the vernier loop technique alone, narrow channel spaced modes may be obtained in one frequency band. However, the larger frequency band with fast switching as required in the cellular band can not be obtained. Consequently, one embodiment of the present invention comprises an improved vernier loop synthesizer in which at least one loop is a fractional-N loop providing cellular frequency spacing.